1. Field of the Invention
The present invention relates to a finder optical system for use in a camera, and more particularly, to a compact real-image zoom finder optical system.
2. Description of the Prior Art
In recent years, because of a reduction in size, an increase in zoom ratio and a popularity of panorama-size photos in lens shutter cameras, a wide-angle and high-zoom-ratio zoom lens for use in a lens shutter camera has been greatly required having a zoom range from wide angle (approximately 28 mm) to middle telephoto (70 mm or greater). This is also required for finder optical systems.
In view of cost, the most advantageous arrangement for a finder objective lens system is a two-unit arrangement of negative, positive configuration. In this arrangement, however, the movement amount of the lens increases as the zoom ratio increases, so that a desired size reduction cannot be achieved. To realize a compact objective lens system, it has been considered necessary to use a positive second lens unit as a strong-power lens including an aspherical surface. However, the manufacture of lenses of such an arrangement is difficult since the aberration sensitivity to errors drastically increases.
Conventionally, as a solution for the achievement of the compactness and high zoom ratio, many finder objective lens systems have been proposed which include many lens units (three unit, four unit or more) or many lens elements (e.g. two units, four elements). For example, Japanese Laid-open Patent Applications Nos. H1-116616, H1-131510, H3-255415, H4-53914, H4-219711 and H4-230719 each realize a high zoom ratio by adding a positive third lens unit to a two-unit arrangement of negative, positive configuration. These prior arts realize the compactness and high zoom ratio by increasing the number of lens elements rather than by providing an aspherical surface in the zoom lens having a strong refractive power. These arrangements, however, is defective since cost increases, ghost due to the reflection of light between surfaces is generated because of the increase in number of lens elements and the finder image is dark due to the reduction in transmittance.